Climate change is any significant change in the measures of climate lasting for several decades or longer, including changes in temperature, precipitation or wind patterns. Historically the Earth’s climate has changed continually, but it is widely agreed that the observed changes over the past 50 years or so have been primarily caused by human activities. How much has the global temperature changed? Long-term air and ocean temperature records clearly show the Earth is warming.

Over the past century, the global air temperature has increased by about 0.8ºC. The oceans are absorbing around 90% of the additional heat, with ocean heat content showing strong increases; on average the temperature of the ocean layer from 0 to 700 metres increased by 0.18ºC between 1955 and 2010. Why does only a few degrees of warming matter?
The Basics of Climate Change. Figure b1.

Greenhouse gases in the atmosphere, including water vapour, carbon dioxide, methane, and nitrous oxide, absorb heat energy and emit it in all directions (including downwards), keeping Earth’s surface and lower atmosphere warm. Adding more greenhouse gases to the atmosphere enhances the effect, making Earth’s surface and lower atmosphere even warmer. Image based on a figure from US EPA. ( larger version)
Climate change: evidence and causes. Project background The Royal Society and the US National Academy of Sciences, with their similar missions to promote the use of science to benefit society and to inform critical policy debates, offer this new publication as a key reference document for decision makers, policy makers, educators, and other individuals seeking authoritative answers about the current state of climate change science.

The publication makes clear what is well established, where consensus is growing, and where there is still uncertainty.
What is the greenhouse effect?
Climate change: How do we know? NASA. The Earth's climate has changed throughout history.

Just in the last 650,000 years there have been seven cycles of glacial advance and retreat, with the abrupt end of the last ice age about 7,000 years ago marking the beginning of the modern climate era — and of human civilization. Most of these climate changes are attributed to very small variations in Earth’s orbit that change the amount of solar energy our planet receives.

Scientific evidence for warming of the climate system is unequivocal. The current warming trend is of particular significance because most of it is very likely human-induced and proceeding at a rate that is unprecedented in the past 1,300 years.1 Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture, collecting many different types of information about our planet and its climate on a global scale.
Climate Change Causes - NASA. Most climate scientists agree the main cause of the current global warming trend is human expansion of the "greenhouse effect"1 — warming that results when the atmosphere traps heat radiating from Earth toward space.

Certain gases in the atmosphere block heat from escaping. Long-lived gases that remain semi-permanently in the atmosphere and do not respond physically or chemically to changes in temperature are described as "forcing" climate change. Gases, such as water vapor, which respond physically or chemically to changes in temperature are seen as "feedbacks.
" Gases that contribute to the greenhouse effect include: Water vapor. On Earth, human activities are changing the natural greenhouse. The consequences of changing the natural atmospheric greenhouse are difficult to predict, but certain effects seem likely: On average, Earth will become warmer.

The role of human activity Solar irradiance Indeed, studies show that solar variability has played a role in past climate changes.
Causes of climate change. What is climate change? CSIRO. Climate change refers to any long-term trends or shifts in climate over many decades.

Weather and climate Weather and climate refer to different aspects of meteorology. Weather is the brief, rapidly changing condition of the atmosphere at a particular place and time, usually changing from hour-to-hour and town-to-town, influenced by the movement of air masses. Climate, on the other hand, is more stable, describing the average weather over at least 30 years.

For example, winter is colder than summer, and Melbourne is colder than Darwin.
Climate change questions and answers CSIRO. Cape Grim greenhouse gas data - CSIRO. The latest greenhouse gas (GHG) data updated monthly from one of the cleanest air sources in the world.

Cape Grim Baseline Air Pollution Station Cape Grim, on Tasmania’s west coast, is one of the three premier Baseline Air Pollution Stations in the World Meteorological Organization-Global Atmosphere Watch (WMO-GAW) network. Baseline stations are defined by the WMO to meet a specific set of criteria for measuring greenhouse and ozone depleting gases and aerosols in clean air environments.

The Cape Grim station is positioned just south of the isolated north-west tip (Woolnorth Point) of Tasmania. It is in an important site, as the air sampled arrives at Cape Grim after long trajectories over the Southern Ocean, under conditions described as ‘baseline’.